Start off by getting some 100, 125, and 145 grain field points. There's no way it should be weak cut to 27.5” with a 100 grain point. If it is still weak, then there might be other problems, like a false weak reading. OTOH, if you find the crossover point where it is not weak, say with a 125 grain point on your 27.5” shaft, then that would be a good indication that you should be shooting .400 spine shafts, so you can use your 165 grain heads and start again with full length 400’s and cut your way down again.